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Bearings under oscillation

Jun 01, 2007

Grease both lubricates and keeps contaminants out of bearings. But its job is often made difficult by back-and-forth, or oscillatory, motion. Printing presses are a classic example: Frequent returns to home coupled with short advances can wreak havoc on lubricants as well as the seals that protect them.

Oscillatory motion also causes another problem: It concentrates and magnifies fatigue on bearing rolling elements and raceways. Under normal operation, stressed contact areas overlap along bearing surfaces. But oscillating motion isolates, rather than distributes, stress, allowing it to build up in localized areas. The cyclic contact accelerates not only bearing fatigue, but also seal wear, especially where shaft finishes are rough, marred, or otherwise compromised. It also tends to trap lubricants, which leads to shearing within local volumes of grease.

Even in these cases, however, long-term lubrication is possible. In fact, there are now systems that allow oscillating machines, even printing equipment, to run for years before relubrication.

How they do it

One solution that's been particularly successful is based on a centralized single-pipe lubrication system. Unlike progressive lubrication, single-pipe systems can be adapted in a modular fashion because they don't rely on a main distributor. More direct lube runs also mean that assembly is easier and most of the overall conduit lengths are shorter. Typical application areas include roller bearings and curve controls, primarily on the operator side.

With cartridges of lubricant optimized for operating conditions, bearing type, and other variables, the right grease can be continuously supplied to the lubrication point. A standard cartridge can deliver measured doses for up to five years. When the lubricant reservoir runs low, another cartridge can be easily swapped in.

Sometimes, fresh grease is pressed directly through relubrication lines to displace used grease, which bleeds out through specially designed repositories. The required amount of lubricant here depends on the geometry and design of the bearing units to be lubricated.

Self-contained lubrication systems can accommodate just about any number and arrangement of bearing points. They also simplify assembly and reduce maintenance work.

Laying the groundwork

To fully leverage the benefits of integrated oscillating roller bearing systems, it is necessary to safeguard long-term grease life.

Main cylinder bearings in most new printing machines are lubricated with oil or grease. This makes it possible to accurately predict lifetime (per application or loading case) for various solutions.

Depending on bearing dimensions and operating conditions (speed, load, temperature) original grease life can be estimated from standard charts. If relubrication is necessary, the L01 interval can be derived from diagrams, using the approximation L01 = L10/2.7, which is valid for selected greases in properly sealed and clean systems.

One way to maximize grease life is by keeping operating temperatures under 40°C or so. This can be done by modifying grease type, filling quantity, and the initial distribution inside the bearing.

Special thanks to SKF for providing information used in this article. For more info, visit skf.com or click onBearingsunder the Component Zone animation atmotionsystemdesign.com.

How used is used?

In a grease, both the base oil and thickener are subject to aging. When the soap structure breaks down, base oil can leak out. Aging also degrades a base oil's ability to form a film, and both of these mechanisms can damage bearings.

There are ways, however, to assess the condition of used lubricating greases. Infrared spectroscopy, for example, analyzes grease additives and their ability to resist oxidation and corrosion. Other tests can measure a grease's ability to protect bearing steel (ISO 11007) and copper (DIN 51811/ASTM D130-04) from corrosion, and its compatibility with polymeric materials, all of which vary with age.

Regarding relubrication

Sooner or later, the grease in a bearing will reach the end of its useful life. When this happens, the bearing must be relubricated. Because grease life is statistical in nature, it's impossible to precisely predict when this will occur. Even so, relubrication intervals may be approximated based on application conditions.

Bearing catalogs tend to be conservative in this regard, typically recommending relubrication cycles every six months or so. These intervals are usually based on L1 service life, an empirical measure that indicates the point at which 99% of the bearings tested are still adequately lubricated. Naturally, it's a good idea to relubricate before the existing grease becomes ineffective — either by hand or with lubrication systems.